The present invention relates to an inkjet recording sheet, and more particularly to an inkjet recording sheet prepared from a cellulosic support such as paper, on which there is applied an inkjet coating providing superior performance.
The most successful inkjet recording sheets presently in use employ non-cellulosic polymer supports because of their exceptional smoothness. However, as the use of inkjet printers becomes more widespread, there is a growing need for developing inkjet sheets using cheaper and more economical substrates such as paper. The use of paper as a substrate for an inkjet recording sheet provides some advantages, such as low cost and the ability to absorb the ink vehicle rapidly during printing, but the main disadvantage is a lack of smoothness as compared with non-cellulosic, polymer substrates.
Inkjet systems are comprised of three components, the printer, the ink and the recording sheet. The printer controls the size, number and placement of the ink droplets and contains the transport system. The ink provides the colorants which form the image, and the recording sheet provides the medium or substrate which accepts and holds the ink. The quality and archivability of ink jet prints is a function of the total system. However, the composition and interaction of the ink and the recording sheet most affect the quality and archivability of the imaged product.
There are two primary requirements for inkjet printing. The first is that the coating, and the substrate in the case of paper supports, must be absorbent enough to immobilize the vehicle of the inks so that the inks will not smear permitting fast ink drying and high printing speeds. The second requirement is that the coating provide a means for keeping the dyes in the inks on the surface of the sheet with minimal spreading, tailing or blurring of dots to provide a sharp image. If the dyes are not kept on the surface of the sheet the colors could fade since the dyes will become diluted by the high light scattering ability of the preferred pigments used in inkjet coatings.
Fast drying properties have been achieved in the past by incorporating silica or other pigments of large specific surface area in the inkjet recording layer so as to increase ink absorption. However, an inkjet recording layer with a pigment of large specific surface area provides a surface having low smoothness. As a result, the appearance of the image deteriorates and the reproduction of the image becomes unsatisfactory. Enhanced smoothness can be achieved, however, by calendering or supercalendering the inkjet recording sheet, but this action tends to destroy the porosity of the inkjet recording layer resulting in a decrease in the ink absorption and reduced drying properties. Nevertheless, emphasis in the prior art has dictated the use of nonflake-like pigments for use in inkjet coatings. Nonflake-like pigments include calcium carbonate, silicas, calcined clays and other such pigments whereas flaky pigments include clays, talc and mica.
Typical binders for inkjet coatings disclosed in the prior art are water soluble and non-water soluble polymeric binders including polyvinyl alcohol, polyvinyl alcohol copolymers such as poly (vinyl alcohol-co-vinyl acetate), hydroxypropyl cellulose, acrylic resins such as poly (methyl methacrylate/ethyl acrylate/acrylic acid), sodium alginate, water soluble phenol formaldehyde resins, carboxylated styrene butadine polymers, carboxymethyl cellulose, hydroxyurethanes, soluble collagen gelatin, hydrolyzed ethylene vinyl acetate polymers, and polysaccharides such as xanthene gum, gum tragacanth, locust bean gum, guar gum, and agur, etc. Also noted in the prior art are aqueous dispersions of poly(vinylpyrrolidone), vinylpyrrolidone-vinyl acetate copolymers, or mixtures thereof. U.S. Pat. No. 4,425,405 discloses such a mixture applied on at least one surface of a paper substrate or incorporated internally of the substrate with a white filler in a pigment-to-binder weight ratio of 10:1 to 0.2:1. In addition, U.S. Pat. No. 4,503,111 discloses the use of poly(vinylpyrrolidone) as the binder in an inkjet recording sheet which uses a hydrophobic substrate prepared from a flexible, transparent plastic material.
However, in accordance with the present invention, a novel coating formulation has been discovered which utilizes many of the components disclosed in the prior art but which produces superior performance when applied to a paper substrate. The present invention is characterized by a careful blending of water soluble binder materials and pigment components to achieve a high level of success.